allow more isolated use of Jekyll::Paginate::Pager

[stevecrozz]

I'm using Jekyll::Paginate::Pager to build tag indexes for my blog: https://github.com/stevecrozz/lithostech.com/blob/master/_plugins/tag_indexes.rb

But since this class references site.config inside ::paginate_path (https://github.com/jekyll/jekyll-paginate/blob/master/lib/jekyll-paginate/pager.rb#L66) I cannot reuse this class for paginating anything that doesn't share the same 'paginate_path.' I propose we modify the ::paginate_path method to accept config as an argument so I can reuse it.

If you agree, I can supply a PR and a test case.

[parkr]

It's all mixed up and crazy. Working on fixing it up in #22, but I don't have the energy and interest right now. Feel free to complete that work if you have free time.